U.S. patent number 4,095,890 [Application Number 05/699,652] was granted by the patent office on 1978-06-20 for xerographic copying apparatus.
This patent grant is currently assigned to Oce-van der Grinten N.V.. Invention is credited to Jozef Marie van Herten.
United States Patent |
4,095,890 |
van Herten |
June 20, 1978 |
**Please see images for:
( Certificate of Correction ) ** |
Xerographic copying apparatus
Abstract
Development of a charged photoconductive medium in xerographic
copying apparatus is prevented when a properly formed charge image
is missing as a result of insufficient imagewise exposure, by a
system in which the intensity of the exposing illumination is
sensed by a detector such as a photoelectric cell or a
phototransistor, and when the illumination is below a level
sufficient to form a proper charge image a signal emitted by a
detector causes a discharging device such as an A.C. corona or a
lamp to discharge the charged medium at a location between the
exposure station and the developing station of the apparatus. Means
also are provided whereby said signal causes interruption of the
feeding of copy material to receive an image, and whereby it causes
the execution of a preset copy program to be blocked until the
exposing illumination is restored to a proper level of intensity.
As a further feature of the invention, the discharging device is a
light source which normally is operated at a reduced intensity for
effecting an integral exposure of the medium to light after the
imagewise exposure, and which is switched to full intensity for
discharging the medium when the illumination for imagewise exposure
is insufficient.
Inventors: |
van Herten; Jozef Marie (Venlo,
NL) |
Assignee: |
Oce-van der Grinten N.V.
(Venlo, NL)
|
Family
ID: |
19824011 |
Appl.
No.: |
05/699,652 |
Filed: |
June 24, 1976 |
Foreign Application Priority Data
|
|
|
|
|
Jun 24, 1975 [NL] |
|
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7507493 |
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Current U.S.
Class: |
399/32; 355/69;
399/50 |
Current CPC
Class: |
G03G
15/04036 (20130101) |
Current International
Class: |
G03G
15/04 (20060101); G03G 015/00 () |
Field of
Search: |
;355/3R,14,69 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Shoop; William M.
Attorney, Agent or Firm: Johnston; Albert C. Dunne; Gerard
F.
Claims
What is claimed is:
1. In a xerographic copying apparatus comprising a photoconductive
imaging medium movable along a processing path having in succession
therealong a device for charging said medium, a station for
exposure of the charged medium to form a charge image thereon, and
a station for developing the charge image, and exposing means
including at least one light source for illuminating an original
and thereby imagewise illuminating the charged medium at said
exposure station, the improvement which comprises means for
detecting the intensity of the illumination issued by said exposing
means and for emitting a signal when said intensity is below a
level sufficient to form on the charged medium a charge image
properly developable at said developing station, and means
responsive to said signal for discharging the charged medium at a
location in said path between said exposure station and said
developing station, thereby preventing development if a proper
charge image is missing.
2. Apparatus according to claim 1, said detecting and signal
emitting means including at least one photoelectric element located
in a path of light from said light source.
3. Apparatus according to claim 2, said photoelectric element being
a photo-electric cell or a photo transistor.
4. Apparatus according to claim 1, said means for discharging said
medium including a discharging device mounted at said location,
means normally keeping said discharging device active when said
medium is being moved along said path, and means responsive to said
detecting and signal emitting means for inactivating said
discharging device when said illumination intensity is at a level
sufficient to form a properly developable charge image on the
charged medium.
5. Apparatus according to claim 4, said discharging device being an
A.C. corona or a light source.
6. Apparatus according to claim 1, said means for discharging said
medium including a discharging device mounted at said location,
said discharging device normally being inactive, and means
responsive to said signal for activating said discharging
device.
7. Apparatus according to claim 6, said discharging device being an
A.C. corona or a light source.
8. Apparatus according to claim 1, and further comprising means at
a transfer station in said path for supplying to said medium
material for receiving a developed image by transfer from said
medium, wherein said improvement further comprises means responsive
to said signal for temporarily rendering said material supplying
means inoperative.
9. Apparatus according to claim 1 and further comprising a control
circuit including a program switch settable for determining the
number of copies to be made of an original, wherein said
improvement further comprises means responsive to said signal for
blocking the execution of a copy program set in said program switch
until said illumination intensity is restored to said sufficient
level.
10. Apparatus according to claim 1, said improvement further
comprising means for integrally exposing said medium to light of
reduced intensity after the imagewise illumination but before
development of the charge image, said means for discharging said
medium being a light source mounted at said location, and means for
normally operating said discharging light source at a reduced
intensity, whereby the same light source serves normally as said
means for integrally exposing said medium and serves in response to
said signal for discharging said medium.
11. In a xerographic copying apparatus comprising a photoconductive
medium movable along a processing path having in succession
therealong a device for charging said medium, a station for
exposure of the charged medium to form a charge image thereon, a
station for developing the charge image into a powder image and a
transfer station having thereat means for supplying to said medium
material for receiving the powder image by transfer from said
medium, exposing means including at least one light source for
illuminating an original and thereby imagewise illuminating the
charged medium at said exposure station, and a control circuit
including a program switch settable for determining the number of
copies to be made of an original, the improvement which comprises
means including at least one photo-electric element located in a
path of light from said light source for sensing the intensity of
the illumination issued by said exposing means and for emitting a
signal when said illumination intensity is below a lever sufficient
to form on the charged medium a charge image properly developable
at said developing station, means responsive to said signal and
including a light source at a location between said exposure
station and said developing station for discharging the charged
medium and thereby preventing development thereof when a proper
charge image is missing, and respective means responsive to said
signal for temporarily rendering said material supplying means
inoperative and for blocking the execution of a copy program set in
said program switch until said illumination intensity is restored
to said sufficient level.
12. Apparatus according to claim 11, and means for normally
operating said discharging light source at a reduced intensity
whereby the same light source serves normally as a means for
integrally exposing said medium to light of reduced intensity after
the imagewise illumination but before development of the charge
image and serves in response to said signal for discharging said
medium.
13. Apparatus according to claim 1, said exposing means including a
plurality of lamps arranged at different locations for illuminating
an original to be copied, said detecting and signal emitting means
including a plurality of light sensitive elements each of which is
located in a direct path of light from at least one of said lamps
and is operative to produce first and second circuit conditions,
respectively, when the intensity of light from the related lamp or
lamps is below and above a required level, and means responsive to
the presence of a said first signal condition in any of said
elements for activating said discharging means.
14. Apparatus according to claim 13, each of said lamps having a
reflector adjoint thereto for reflecting light from the lamp toward
the original, each of said reflectors having one of said
light-sensitive elements mounted therein at a location therein from
which light from the lamp is not reflected to the original.
Description
This invention relates to a xerographic copying apparatus of the
kind in which a photoconductive medium is transported successively
past a charging device, an exposing station and a developing
station.
Apparatus of that kind is generally known, for instance as shown in
U.S. Pat. No. 2,357,809. With the known apparatus, after charging
the photoconductive medium the image of an original is projected
onto the electrically charged surface to form on it a charge image
which is converted into a powder image in the developing station.
This powder image is subsequently transferred in a transfer station
to a receiving material on which the powder image is fixed, while
the photoconductive medium is regenerated for reuse.
If the light source at the exposure station of the known apparatus
does not function correctly, for instance because of one of the
lamps being defective, or if the feed unit fails, the
photoconductive medium will not be discharged imagewise or will be
discharged insufficiently. This results in an unusable image being
produced in the developing station, which image shows either a
black surface or dark background portions, but it is also
particularly detrimental to the regeneration of the photoconductive
medium. The reason is that, following the transfer of the powder
image to the receiving material in the transfer station, a residual
image is left on the photoconductive medium to be removed by a
cleaning device which forms part of the regeneration device.
Depending upon the transfer method used, the residual image
represents 5 to 30 percent of the weight of the original powder
image.
It has been observed that when a photoconductive medium is not
exposed after being charged, the quantity of powder transferred
onto the photoconductive medium in the developing station is 20 to
30 times greater than in the case of a normal imagewise exposure.
Consequently, any image residue left on the medium will be 20 to 30
times heavier, so that either the cleaning device must have been
adapted to overcome these extreme conditions or the service life of
the photoconductive medium, i.e., the number of copies that can be
made with it, decreases considerably.
The object of the present invention is to provide in a xerographic
copying apparatus a system by which the disadvantages above
mentioned are prevented.
According to the invention this object is attained by providing the
apparatus with at least one detector which senses the intensity of
the illumination resued by the means, including at least one light
source, that serve for exposing the original and thus forming the
charge image in the productive medium at the exposure station,
which detecting means emits a signal when the illumination
intensity is below a lever sufficient to form a proper charge
image, together with a discharging device which acts on the charged
photoconductive medium in response to said signal, at a location
between the exposure station and the developing station, so as to
discharge the medium and thus prevent development if the medium is
improperly exposed or is not exposed at all.
In this way, the already charged photoconductive medium is
discharged again if the exposing means does not function at all or
does not function correctly, so that no powder is attracted by the
photoconductive medium in the developing station. As a result, it
suffices to use a cleaning device of less robust construction,
enabling the cleaning device to be simpler and less costly and the
service life of the photoconductive medium may be increased.
The detector or detectors mentioned may each be a light-sensitive
element, such as a photocell or a phototransistor, that responds to
the light emitted by the light source, or may be an element that
responds to the electric current through the light sources.
According to a further feature of the invention, the copying
apparatus is provided with a means whereby the supply of receiving
material to the transfer station, where the image is transferred
from the photoconductive medium onto a receiving material, is
temporarily interrupted when the discharging device is activated.
Consequently, there is no needless supply or waste of receiving
material.
A further feature of the invention as applied to a copying
apparatus provided with a program switch for selecting the number
of copies to be made from an original, consists in that means are
provided for interrupting the execution of the copy program when
the discharging device is activated, and until the exposure again
functions normally.
A particularly advantageous embodiment of the invention is obtained
when the exposure of the photoconductive medium after charging is
divided into an integral exposure and an imagewise exposure, as
described e.g. in British patent specification No. 1,284,887, and
the discharging device is constituted by a lamp. It has been found
that the integral exposure can be effected satisfactorily after the
imagewise exposure, by means of a lamp operated at reduced
capacity, and this same lamp can serve for discharging the medium
by being switched to full capacity if there is a failure of or an
improper imagewise exposure. The effect obtained by applying
imagewise exposure first and integral exposure subsequently does
not appear to differ significantly from the effect obtained by
applying integral exposure first and imagewise exposure
subsequently as described in said British patent specification.
The above mentioned and other features and advantages of the
invention will be apparent from the following description and the
accompanying drawings of illustrative embodiments of the invention.
In the drawings:
FIG. 1 is a schematic sectional view of an electrophotographic
apparatus provided with means for discharging the photoconductive
medium according to the invention;
FIG. 2 is a section taken along line II--II of FIG. 1;
FIG. 3 is a partial section, at enlarged scale, taken along line
III--III of FIG. 2;
FIG. 4 is a diagram of a circuit for coupling the detector with the
discharging device;
FIG. 5 is a diagram of a modification of the circuit of FIG. 4;
and
FIG. 6 is a diagram of a further modification of the circuit of
FIGS. 4 and 5.
The various parts of a copying apparatus in which the invention can
be utilized to advantage are represented schematically in FIGS. 1
and 2.
In the apparatus there shown, an original to be copied is laid on a
transparent glass plate 1 and pressed down against the glass plate
by a cover 2, and then is exposed by means of four flashlamps 3.
The image reflected by the original is projected, via an optical
system consisting of a lens 4 and mirrors 5 and 6, onto a part of
the photoconductive medium 8 present in the projection station 7,
which photoconductive medium is an electrophotographic element or
plate in the form of an endless belt.
The endless belt 8 is transported in the apparatus via a number of
rollers, with an interposed period of storage of a length of the
belt in a magazine 9, according to the system described in U.S.
Pat. No. 3,926,625. The length of the belt extending from and back
to the magazine is moved with a constant speed in the direction of
the arrows.
The photoconductive layer of the belt is charged before exposure,
by a corona charging device 10. The projected light image
discharges the parts of the photoconductive layer struck by the
light, forming on the belt a latent electrostatic image which
corresponding with the original. Upon further moving of the belt
the latent electrostatic image passes a developing station 11,
where developing powder is brought into contact with the belt
surface with the aid of a so-called magnetic brush 12, in order to
develop the latent image and convert it into a powder image.
The power image in transported by the belt to a transfer station
13, where it is brought into contact with a sheet of copy paper
that is moved forward with the same speed as the belt and onto
which the powder image is transferred, for instance by applying a
suitable electric field. A sheet feeding device 14 is provided for
transporting copy sheets individually and successively into contact
with the belt in the transfer station.
The sheet that receives the powder image is separated from the belt
8 at the transfer station and passed through a fixed device 15 by
which the transferred powder image is fixed onto the sheet of copy
paper. Then the sheet is passed into a receiving tray 16 opening
outside of the apparatus.
Any part of the power image which is not transferred to the sheet
of copy paper is transported beyond the transfer device 13 with the
belt 8 and past a cleaning device 17 by which the residual powder
is removed.
The copying apparatus further comprises suitable drive means and
guide means for driving the belt 8 in timed relationship with the
flash exposure of an original to be copied, for separating and
supplying sheets of copy paper and transporting these through the
transfer device 13, and for transporting a sheet of paper through
the fixing device 15 to the receiving tray 16.
The above description is illustrative of the general operation of a
type of electrostatic copying apparatus in which the present
invention can be utilized to advantage. The invention, however, may
also be utilized for other forms or types of apparatus of the kind
first mentioned hereinabove.
According to the invention, a discharging device 20 is provided
adjacent to the transport path of the photoconductive belt at a
location between the projection station or exposure plane 7 and the
developing device 11. The discharging device 20 may be, for
example, an A.C. corona discharging device or a light source such
as a glow lamp.
Further, at least one detector 21 for sensing the functioning of
the lamps 3 is installed according to the invention in the exposing
station (see FIG. 3). When the detector utilized is a photocell or
photoresistor, it may be installed, for instance, beside the lens 4
or beside the glass plate 1. In the embodiment illustrated, in
which four flash lamps 3 are used for the exposure and each lamp is
provided with a reflector 22, a detector 21 is provided for each
lamp and is installed in a part of pertaining reflector 22 at a
location where the reflector is considered as ineffective because
the rays reflected by that part of it do not or only slightly
expose the glass plate 1, being mainly absorbed by the lamp 3
itself.
The discharging device 20 and the detector or detectors 21 are
electrically coupled in a manner dependent upon the manner of
operation of the copying apparatus. A preferred manner of coupling
them is described below in more detail. In any case, the electric
coupling is such that, upon insufficient exposure of the glass
plate because one of the lamps does not function or functions
badly, so that the corresponding portion of of an original on the
exposure plate 1 and, consequently, the belt 8 is exposed
insufficiently, the charge on that portion of the belt 8 is
neutralized fully by means of the discharging device 20 when the
relevant belt portion moves past this device.
The means coupling the discharging device and the illumination
detector or detectors may form part of or may be included in the
electric control circuit of the copying apparatus. FIG. 4 shows a
circuit for coupling two detectors 21 (one for each pair of
flashlamps) with a discharging device 20, which circuit is suitable
for being connected with the control circuit described in detail in
U.S. Pat. No. 3,912,390.
As described in U.S. Pat. No. 3,912,390, when making one copy the
corona charging device 10 is switched on and off. At a certain
moment after charging up, namely, when the charged part of the
photoconductive belt 8 has arrived in the exposure plane, a flash
pulse is formed, as a result of which the lamps 3 emit a light
flash.
In the circuit of FIG. 4, the detectors 21 are each connected in
series with a potentiometer 30 which is connected with the +5V
terminal of a voltage supply source. When light falls upon a
detector 21, the detector becomes conductive and a voltage decrease
occurs over the potentiometer 30, so that the voltage in the
connection lead between 30 and 31 becomes lower than +5V. Each
connection lead is connected with one of the inputs of a NOR-gate
31, so that when sufficient light falls on all detectors 21, a
signal pulse is formed in the output of the NOR-gate 31. By means
of the potentiometer 30 it is possible to adjust the minimum
intensity of light required to generate an effective signal in the
inputs of the NOR-gate 31.
The flash pulse already mentioned and the signal of the NOR-gate 31
are combined in a NAND-gate 32, the output of which temporarily
becomes zero when the detectors 21 have registered sufficient
light. This temporary zero condition of the output of the gate 32
is used for setting a flipflop 33, the output 34 of which is used
for opening a counter 35. This counter 35 is built up of a number
of JK-flipflops, for instance of the type Texas Instruments SN
7473, the functioning of which is generally known. The clock input
of this counter is connected with the pulse counter described in
the above mentioned U.S. Pat. No. 3,912,390. After the counter 35
has been opened via a signal of the flipflop 33, these pulses are
counted. The outputs of counter 35 are connected with a decoding
circuit 36 which is adjusted on two fixed numbers A and B. These
numbers are so chosen that when the number A is reached in the
counter 35, the leading edge of the charged part of the belt
reaches the discharging device 20, and when the number B is reached
in the counter 35 the trailing edge of the charged part of the belt
has passed the discharging device 20.
When in the counter 35 the number A is reached, a signal pulse is
generated in the output 37 of the decoding circuit 36. The output
37 is connected via an invertor 39 with the reset input of a
RS-flipflop 41. When the number B is reached in the counter 35, a
signal pulse is generated in the output 38 of the decoding circuit
36. The output 38 is connected via an invertor 40 with the set
input of the flipflop 41. The output 42 of the flipflop 41 is
connected via an amplifier 43 with a switching element 44, with
which the discharging device 20 can be switched on and off.
In normal conditions, so at satisfactory functioning of the
exposure lamps, the operation of the device is as follows:
At the moment when a flash pulse is generated, the lamps ignite, so
that at the same time the phototransistors 21 are exposed and
consequently become conductive. In the output of the NOR-gate 31 a
signal pulse is generated which is combined with the flash pulse in
the NAND-gate 32, in the output of which a signal pulse is
generated. By this signal pulse the flipflop 33 is set so that in
the output 34 a signal is generated, by which the counter 35 is
opened.
As soon as the number A is reached in the counter 35, a signal
pulse is generated in the output 37 of the decoding circuit 36,
which signal pulse resets the flipflop 41 via the invertor 39, so
that in the output 42 no signal is present. Thus the switching
element 44 is no longer excited and the discharging device 20 is
switched off. As soon as the counter 35 has reached the number B, a
signal pulse is generated in the output 38 of the decoding circuit
36, which pulse sets the flipflop 41 via the invertor 40. A signal
is thus generated in the output 42, which signal re-excites the
switching element 44 via the amplifier 43. The output 38 is also
connected with a NAND-gate 45, which is also connected with the
clock pulse generator, so that when the number B is reached, the
flipflop 33 is reset, whereby the counter 35 is reset and is
blocked.
In case one of the phototransistors 21 is not exposed or is exposed
insufficiently, no signal is generated in the output of the gate
31, so that the counter 35 cannot be opened, which means that the
discharging device 20 keeps working and the photoconductive belt is
fully discharged.
FIG. 5 shows a second embodiment of a circuit suitable for coupling
the detectors 21 and the discharging device 20, by which circuit
the discharging device 20 is activated only when this is
necessary.
Just as in the circuit of FIG. 4, the detectors 21 are each
connected in series with a potentiometer 30. Each connection lead
is connected with an invertor 50 or 51, the outputs of which are
combined in a NAND-gate 52. The gate 53, the flipflop 54, the
counter 56 and the decoding circuit 57 corresponding respectively
to the elements 32, 33, 35 and 36 of the circuit of FIG. 4. A
signal is generated in the output 59 of the decoding circuit 57
when the counter reaches the number A, and a signal is generated in
the output 58 of the decoding circuit 57 when the counter reaches
the number B. Via the invertors 60, 61, a flipflop 62 and an
amplifier 63 these signals can be used for switching the
discharging device 20 on and off via the switching element 64.
In normal conditions, so at satisfactory functioning of the
exposure lamps, the operation of the device is as follows:
At the moment when a flash pulse is generated, the lamps ignite, so
that at the same time the phototransistors 21 are exposed and thus
become conductive. Thus the generation of a signal (a U-shaped
signal pulse) in the output of the gate 52 is temporarily
interrupted, so that the gate 53 is blocked and nothing further
happens. The discharging device 20 continues to be switched
off.
When at least one of the detectors 21 is not exposed or is exposed
insufficiently, the output of the gate 52 continues to be high, so
that the flash pulse is transmitted by the gate 53, and the
flipflop 54 is set; so the counter 56 is opened via the output 55.
When the number A is reached, a signal is generated in the output
59 of the decoding circuit 57, which signal sets the flipflop 62
via the invertor 61, whereby the switching element 64 is excited
via the amplifier 63 and the discharging device 20 is switched on.
When the number B is reached, the flipflop 62 is reset via the
output 58 and the invertor 60; so the excitation of the switching
element 64 is stopped, and the discharging device 20 is switched
off. At the same moment the flipflop 54 is reset via the NAND-gate
65, by which the counter 56 is reset and blocked.
A coupling circuit of the type described above can also be provided
in other ways. For instance, instead of the flipflop 33 or 54, the
counter 35 or 56 with decoding circuit 36 or 57 and the flipflop 41
or 62, two parallel-switched delaying elements can be used, which
cause signal delays in proportion to the numbers A and B, whereby
the switching element 44 or 64 is connected with these delaying
elements and is switched off or on via the delaying element A, and
is switched on or off via the delaying element B.
The signal which is generated when at least one of the exposing
lamps does not function, or functions insufficiently, can also be
used to advantage according to the invention for protecting other
functions of the copying apparatus, especially the feeding in of
copy sheets and the execution of a copy count by a program control
switch. This is an advantageous feature in that, as will be
apparent, if the discharging device has functioned in the safety
system and nothing further happens, yet a sheet of copy paper is
transported through the transfer station and the fixing device, a
blank sheet will be delivered as a copy from the apparatus, and it
will be counted as a copy.
An electric circuit with which such delivery of a blank copy sheet
can be prevented is shown in FIG. 6. By this embodiment, not only
the protective function of the discharging device 20 is achieved
but also, after the exposing lamps have not functioned at all or
have functioned insufficiently, the introduction of a copy sheet
and counting of this sheet as a copy by the program switch are
prevented.
In the embodiment of FIG. 6 it is presupposed that the discharge
device is to be normally and continuously excited, as in the first
described embodiment, and further that a signal referred to
hereinbelow as a "copy sheet" signal is generated in the apparatus
at a certain moment, by which signal a sheet of paper is caused to
arrive at an exact moment in the transfer station, and a "copy"
signal is then excited which is led to the program switch and
causes it to register that a copy is coming or has been made. These
signals can be formed, for instance, as described more particularly
in the said U.S. Pat. No. 3,912,390. Further it is supposed for
purposes of the circuit shown that only one detector is used in the
exposure station.
The detector 21 is connected in series with a potentiometer 30, and
their connecting lead is connected via an invertor 70 with a
NAND-gate 71 to which the flash pulse is also supplied. The output
of the gate 71 is connected with the set input of an RS-flipflop
72, the output 73 of which is used for opening a counter 74 the
outputs of which are connected with a decoding circuit 75. All this
is as described in relation to FIG. 4
The decoding circuit in the embodiment of FIG. 6 is set on six
numbers A, B, C, D, E and F. The numbers A and B have the same
significance as described above with relation to FIGS. 4 and 5. The
numbers C and D correspond with the moments at which, in normal
operation of the apparatus, the "copy" signal starts or ends,
whereas the numbers E and F correspond with the moments at which,
in normal operation of the apparatus, the "copy sheet" signal
starts or ends.
When the numbers A, B, C, D, E and F are reached, signals are
respectively generated in the outputs 76, 77, 78, 79, 80 and 81.
The outputs 77 and 76 are connected via respective invertors 82 and
83 with respectively the reset and set inputs of the flipflop 84,
the output of which is connected via an amplifier 85 with a
switching element 86 for switching the discharging device 20 on and
off, all as described with relation to FIG. 4.
The output 78 is connected via an invertor 87 with the set input of
a flipflop 89, the reset input of which is connected via an
invertor 88 with the output 79 of the decoding circuit 75. The
output 95 of the flipflop 89 is combined with the "copy" signal in
a NAND-gate 90 so that the "copy" signal is blocked when no signal
is present in the output 95. NAND-gate 90 is connected with the
program switch via an invertor 96.
The output 80 is connected via an invertor 91 with the set input of
a flipflop 93, the reset input of which is connected via an
invertor 92 with the output 81 of the decoding circuit 75. The
output 97 of the flipflop 93 is combined with the "copy sheet"
signal in a NAND-gate 94, so that the gate 94 blocks the "copy
sheet" signal when no signal is present in the output 97. The gate
94 is connected via an invertor 98 with the sheet feed-in
mechanism.
The output 81 of the decoding circuit 75 also is combined in a
NAND-gate 99 with the clock pulse, and the output of the gate 99 is
connected with the reset input of the flipflop 72. Hereby it is
accepted that the number F is the greatest of the six numbers A to
F.
The normal operation of the system is as follows, under conditions
existing when the exposure functions correctly. The phototransistor
21 becomes conductive, so that the gate 70 generates a signal pulse
which is combined in the gate 71 with the flash pulse, by which the
flipflop 72 is set and the counter 74 is opened via the output 73.
When the number A is reached, a signal is generated in the output
76, which signal resets the flipflop 84 via the invertor 83, so
that in its output no signal is present and the discharging device
20 is switched off via the amplifier 85 and the switching element
86. Subsequently the number B is reached in the counter, causing a
signal by which the flipflop 84 is set via the output 77 and the
invertor 82, and the discharging device 20 is again excited. When
the number C is reached, the signal sets flipflop 89 via the output
78 and the invertor 87, and the gate 90 is opened, so that the
"copy" signal is transmitted. The flipflop 89 is reset via the
output 79 and the invertor 88, when the number D is reached. When
the number E is reached, the flipflop 93 is set via the output 80
and the invertor 91 and a signal is generated in the output 97,
which signal opens the gate 94 so that the "copy sheet" signal is
transmitted. The flipflop 93 is reset via the output 81 and the
invertor 92 when the number F is reached.
When the number F is reached, a signal pulse also is generated in
the output 81 connected with the gate 99, so that the flipflop 72
is reset via the gate 99 and the counter 74 is thus reset and
blocked.
When the exposure does not function correctly, the flipflop 72 is
not set, and the counter 74 therefore stays blocked. Consequently
the gates 90 and 94 are also kept blocking, and the "copy" and
"copy sheet" signals are not transmitted.
It will be apparent that other functions in the apparatus can also
be protected in the same way, such, for example, as the application
of voltage in the transfer station and the switching-on of the
developing device. Further it is obvious that the circuit
arrangement of FIG. 6 can easily be changed into one suited for the
use of a discharging device that does not operate
uninterruptedly.
Another advantageous application of the invention is found in
connection with the principle described in British patent
specification No. 1,284,887, in which it is proposed that the
photoconductive medium, after having been charged, be first exposed
integrally with limited intensity and subsequently exposed
imagewise, in order thus to influence the gradation of the image
formed. It has been found that it makes no difference in this
regard if the integral exposure with limited intensity takes place
after, instead of before, the imagewise exposure has taken place.
Accordingly, when a light source is employed as a discharging
device according to the present invention, this light source can
serve for discharging the photoconductive medium, by being switched
to its full illuminating capacity, or for effecting the integral
exposure with limited intensity by being switched to operate at a
suitable reduced capacity. In this case the switching element 44,
64 or 86 of a circuit such as that of FIG. 4, 5 or 6 is used for
switching the light source to full capacity or to reduced
capacity.
The invention of course is not restricted to the particular
embodiments described and illustrated in the drawings, as numerous
modifications can be employed, and especially so in relation to the
circuit between an exposure detector such as detector 21 and a
device for discharging the photoconductive medium, such as device
20, while still employing the principle of this invention.
* * * * *